ProMods

You didn't connect them properly (red-red).

You MUST connect between green-green or red-green. You cant connect red-red. To change place of red node use other origin. ALWAYS connects with ALT clicked. It'll give you 100% sure of correct connection.

Thank you for your Answers
I connected it red-green and green-green and it didn't work.
I fixed the problem by myselfe ... I had to create new roads at the right end of the prefabs

But thank you for your Support

Right, sometimes connection prefab-prefab not correct properly (f.e. holes between prefabs). In that case you should place short road as something like patch.

You could also check all the connections and use ALT key while using the move tool to force the connections.

Going back to the semaphore question: Say I want a German one and it only comes up a Dutch one, how do I get that country's one to appear?

If you place a prefab in Germany and enable semaphores - German traffic lights will appear. It depends on the country.

This one I can't solve. How can I 'paint' the texture on a gas station prefab? I want to go from a dirt texture to a grass texture.

You should have this option on 'Node properties'. Try to change something like grass, or default to option like desert. I dont on map editor so I cant say more exacly.

You can't use brush on prefabs, unfortunately.